How to Calculate Pogo Pin Spring Force (With Worked Examples)

Engineering Note #1 — Jiatel Hardware Electronics

Most pogo pin failures are not manufacturing failures. They are specification failures: a pin was selected on the wrong number, at the wrong point in its travel, for an environment nobody calculated.

This note gives you the equations, the failure modes, and the decision tables you need to specify spring force correctly. It does not give you a catalogue of our force values, because we do not build to a catalogue — see the last section for why.

1. Three forces that get confused

Engineers routinely use “spring force” to mean three different things. They behave differently, and they are measured differently.

Force	What it is	Who it acts on	Typical spec source
Contact force	The force at the plunger tip against the mating pad, at the working deflection	The electrical interface	Design calculation
Insertion / withdrawal force	Total force to mate or unmate the whole connector	The housing, the operator, the mating hardware	EIA-364-13
Retention force	Force holding the pin body in its housing or the connector on the board	The mechanical joint	EIA-364-35 (insert retention)

A single-pin connector rated at 60 g contact force can still need 400 g to insert, because insertion force scales with pin count and includes the housing and any latch. Conversely, a connector that inserts smoothly can fail in the field because contact force at the working deflection was never the number anyone checked.

Rule: specify contact force at the deflection your application actually uses — not at full compression, not at zero.

2. The governing equation

For a coiled spring inside a pogo pin:

F_contact = F_preload + k × δ

Where:

F_preload — force at the pin’s free (uncompressed) position, sometimes called initial force
k — spring rate, in grams-force per millimetre (gf/mm) or newtons per millimetre (N/mm)
δ — deflection from free position, i.e. how far the plunger is pushed in

Two consequences follow immediately, and both are frequently missed.

First: the force at zero travel is the minimum force, not the design force. If you specify only F_preload, you have specified the worst case and called it the nominal.

Second: the force is linear in deflection, but the useful range is not. A pogo pin has a rated working travel, usually around half of maximum stroke, and a maximum stroke beyond which the spring reaches solid height. Pushing past maximum stroke does not increase contact force usefully — it bottoms the spring and transfers load into the barrel and the solder joint.

3. Where the design point actually is

This is the single most useful thing to understand about published spring force data, and it is where most design reviews go wrong.

Spring-loaded pin datasheets conventionally publish two force figures:

Initial force (pre-load) — measured at free position
Force at mid-stroke — measured at half of maximum stroke

Mill-Max, whose datasheets are the de facto reference format in this industry, publishes exactly this pair. Their Farnell-distributed spring-loaded pin datasheet lists Initial Force (Pre-Load), Force @ Mid. Stroke, Mid. Stroke and Max. Stroke as separate columns, and their high-current small-scale series is specified as 60 grams at mid-stroke with an 8 A rating (6.4 A derated) at a maximum pin diameter of 2.1 mm (0.083 in).

Now consider what happens in a typical application. You pick a pin whose mid-stroke force is 60 g. You mount it so the plunger is deflected by only 40 % of maximum stroke instead of 50 %. Your actual contact force is lower than 60 g — and if the pin is a short-stroke type, it may be close to the pre-load figure.

The design point is neither published number. It is F_preload + k × δ_actual, and you have to compute it for your stack height.

If the datasheet gives you initial force and mid-stroke force, you can recover the spring rate without asking the supplier:

k = (F_mid − F_initial) / δ_mid

Then evaluate at your real deflection. This is the calculation that should appear in every design review, and almost never does.

4. The failure nobody calculates: vibration lift-off

Contact force has to beat more than the spring’s own pre-load. In a vibrating environment it also has to beat the inertial force of the moving parts.

The moving mass in a pogo pin is small — plunger plus the active part of the spring — typically well under a gram. But acceleration in automotive and industrial environments is large, and the comparison is not intuitive.

The lift-off condition is:

F_contact > m × a_peak

Worked example. A plunger-and-spring assembly with moving mass m = 0.3 g, in an application seeing 10 g RMS random vibration. Taking the peak as roughly 3σ ≈ 30 g for a conservative check:

F_liftoff = 0.3 g × 30 = 9 gf

A pin designed for 60 g at mid-stroke has a margin of 60 / 9 ≈ 6.7×. That is comfortable.

Now run the same pin at 40 % deflection, where the computed contact force might be 45 g, in an environment with occasional 50 g shock:

F_liftoff = 0.3 g × 50 = 15 gf
margin = 45 / 15 = 3×

Three times is the edge of comfortable. Below about 3×, micro-motion between plunger and pad starts to matter: fretting corrosion builds on the contact surfaces, and the failure appears as intermittent open circuits rather than a clean high resistance. Intermittent opens are the most expensive failure mode in the field because they do not reproduce on the bench.

Note on the numbers: m and a are yours to supply. We are not giving you a force value here — we are giving you the equation and the margin criterion. Substituting your own mass and vibration profile takes two minutes and is the difference between a specified joint and a guessed one.

5. Contact resistance is not linear in force

Engineers often assume halving contact force roughly doubles contact resistance. It does not.

For a clean metal-to-metal contact, constriction resistance falls approximately with the square root of contact force (Holm’s classical treatment of contact resistance). Going from 60 g to 30 g raises resistance by roughly 40 %, not 100 %.

This matters for two reasons:

A modest force reduction does not fail immediately on resistance. The joint passes end-of-line test and fails in the field. Resistance-based acceptance testing will not catch a marginal force design on the first unit.
The real risk is film breakthrough, not bulk resistance. At low force the plunger may not reliably break through oxide or contamination. The result is a contact that is fine when measured and open when vibrated — again, the intermittent failure.

Practical consequence: do not specify minimum contact force from a resistance target alone. Specify it from the vibration margin and the film-breakthrough requirement, then verify resistance as a secondary check.

6. Current, temperature, and force relaxation

A pogo pin carrying current gets hot. The heat does two things to the spring:

Reduces the shear modulus of the spring material, so force at a given deflection drops
Accelerates stress relaxation, a permanent loss of force that accumulates with time at temperature

Stress relaxation is the one that surprises people. A spring held compressed at elevated temperature for a long period will not return to its original free length. The force loss is time-and-temperature dependent and is not recovered when the current is removed.

This is why a pin that passes a 60 g check at 25 °C can measure 45 g after a thousand hours at 125 °C. The design margin you computed in section 4 has to survive that drift, not just the day-one value.

Design sequence that accounts for it:

Compute required contact force from vibration margin and film breakthrough
Add the expected force loss over life at operating temperature (from the spring material’s relaxation data, not from a rule of thumb)
Add a margin for manufacturing spread in spring rate
Specify the resulting number as a minimum at end of life, not a nominal at time zero

7. Decision matrix 1: environment → dominant failure

Application environment	Dominant failure mode	Governing criterion	What to nail down with the supplier
Indoor, fixed equipment	Contact resistance drift	Contact force stays above film-breakthrough threshold for life	Plating material and thickness; maximum contact resistance
Handheld / portable	Lift-off during drop	`F_contact > m × a_impact`	Moving mass of plunger and spring; peak drop acceleration
Automotive / heavy vibration	Fretting corrosion, intermittent open	`F_contact > m × a_rms × safety factor`, factor ≥ 3	Vibration profile; how the supplier defines safety factor
High mating cycles	Spring fatigue, travel loss	Minimum contact force still met after rated cycles	Rated cycle count; force-versus-cycles data
High current	Thermal softening and relaxation	Current × contact resistance = temperature rise; rise reduces force	Current derating curve; allowable temperature rise; spring material temperature limit
Fine pitch, limited space	Insertion force too high for the housing	Total insertion force within housing and operator limits	Force per pin at your deflection; pin count

8. Decision matrix 2: what to put in an RFQ

Most pogo pin RFQs fail not because the supplier cannot build the part, but because the request omits the variables that determine whether the part works. Send these seven items and you will get a usable quote instead of a price list.

#	Item	Why it matters
1	Working deflection in mm (not just “compressed”)	Sets the design point; without it, any quoted force is meaningless
2	Required contact force at that deflection, minimum, at end of life	Separates nominal from minimum
3	Vibration and shock profile (g, frequency band, duration)	Determines the margin criterion
4	Current per pin and allowable temperature rise	Drives the derating calculation
5	Mating cycle count over product life	Determines plating and spring selection
6	Plating requirement (material, thickness, or the corrosion test you must pass)	Affects both resistance and wear
7	Mechanical envelope: pitch, barrel diameter, maximum height, board thickness	Constrains which spring can physically fit

An RFQ missing items 1 and 2 is the most common case, and it is the one that produces a part which measures correctly on the bench and fails in the enclosure.

9. How Jiatel specifies spring force

We do not publish a spring force catalogue, and that is a deliberate engineering position rather than a gap in our documentation.

The reason is section 3: the correct force is F_preload + k × δ_actual, evaluated against a vibration margin, a thermal relaxation allowance and a life requirement. Two customers using the same barrel diameter with different stack heights, different vibration profiles and different life targets need different springs. A single catalogue number would be right for at most one of them.

So our specification process is:

You give us the seven RFQ items in matrix 2
We select the spring, or design one, against your deflection and margin
We state the force at your working deflection, as a minimum at end of life
The value is verified by dynamic testing on the built part, not asserted from a datasheet

Final spring force is defined per custom application requirements and verified through dynamic testing. If a supplier quotes you a single force number without asking for your deflection, they are quoting a spring, not a joint.

10. What to do next

If you are specifying a pogo pin interface now, the fastest path is:

Compute your required contact force with the vibration margin in section 4
Check whether the pin you have selected reaches it at your deflection, not at mid-stroke
Send us the seven RFQ items — we will return a force specification at your working point

→ Request 2D Drawing & Pinout — send your stack height and mating pad geometry, and we will confirm the deflection and the force that follows from it.

→ Request a Custom Pogo Pin Quote — include the seven items from matrix 2 and we can quote to a specification rather than a drawing.

References

EIA-364-13 — Mating and unmating force test procedure for electrical connectors. Force measured at a rate not exceeding 25.4 mm/min.
EIA-364-35 — Insert retention test procedure, axial force.
Mill-Max spring-loaded pin datasheets — published convention of Initial Force (Pre-Load), Force @ Mid. Stroke, Mid. Stroke and Max. Stroke as separate figures. High-current small-scale series specified at 60 g mid-stroke, 8 A (6.4 A derated), max diameter 2.1 mm / 0.083 in.
Holm, R. — classical treatment of constriction resistance at metal contacts; resistance varies approximately with the inverse square root of contact force.

All force values in this note are either computed from stated assumptions or quoted from the cited sources. No Jiatel catalogue value is asserted, because none exists by design.